The chrominance (chroma) component of conventional broadcast video signals include, in sequential format, a synchronizing color burst reference signal followed by color image information. The amplitude of the color burst and the ratio of the amplitude of the color burst to the amplitude of the image information are generally fixed by convention. Not infrequently, the magnitude of the color burst (and the image information) of the received signal deviates from the desired level due to faulty broadcast equipment or the transmission medium, etc. To compensate for these deviations and restore the chrominance signal to nominal levels, conventional receivers include automatic chrominance control (ACC) circuits. The ACC circuits compare the burst magnitude to a preset reference and amplify or attenuate the chrominance signal to maintain the burst signal amplitude constant at the desired level.
Due to faulty ACC operation or differential color burst-image information deviations, the ACC circuit may raise the chrominance signal magnitude undesirably high. The effect of this is to reproduce images with overly saturated colors. To compensate for this latter contingency, chroma overload circuitry is provided which monitors the chrominance signal after it has passed through the ACC circuitry, and attenuates the chrominance signal when its magnitude exceeds a predetermined amplitude.
The occurrence of chroma overloads, i.e. chroma signal exceeding a predetermined amplitude, normally correspond with portions of the chroma signal representing images with highly saturated color. In a field or a frame period, the percentage of chroma overload will be a function of the particular image displayed. If a chroma overload condition exists, only a small portion of a frame period may contain chroma signal exceeding the desirable amplitude (overload). However, the amplitude of the chroma signal over a greater portion of a field or frame period, while not exhibiting overload, may be proportionately larger than desired. In other words, whatever created the overload condition in the more highly saturated image portions of a frame period probably proportionately affects the entire chroma signal. Thus, for chroma overload correction, it is generally desirable to generate a correction signal which is applied to, e.g. a large portion or the entirety of the successive field or frame of chroma signal, and not to just the areas exhibiting overload conditions.
It has also been determined to be desirable to design chroma overload systems with faster attack times than decay times. The attack time is the time over which the chroma signal is attenuated in response to the detection of overload conditions. The decay time is the time over which chroma signal attenuation is reduced in the absence of overload conditions.